iru431l / iru431al 1 rev. 1.5 07/13/01 typical application description the iru431l is a three-terminal adjustable shunt regu- lator that can also be used as a precision voltage refer- ence. its output voltage may be set to any value be- tween vref(1.24v) and 15v with two external resistors as shown in the typical application circuit. other appli- cations of this device include being used as a merged amplifier and reference in applications such as a linear regulator or as the secondary side controller in low volt- age power supply applications. the iru431l only re- quires 80 a maximum quiescent current before regu- lating, making it ideal as a voltage reference for battery type applications. low -voltage adjustable precision shunt regulator package order information ta ( c) 5-pin pkg 3-pin pkg 8-pin plastic sot-23 (l5) mark sot-23 (l3) mark soic (s) 0 to 70 iru431lcl5 431l iru431lcl3 431l iru431lcs 0 to 70 iru431alcl5 431a iru431alcl3 431a IRU431ALCS figure 1 - typical application of the iru431l as a shunt regulator / voltage reference. data sheet no. pd94118 r b r1 r2 co iru431l v in v out features sot-23 and soic packages 0.5% voltage reference initial accuracy (iru431al) low operating cathode current (80 a max) unconditionally stable with only 1 f adjustable output from 1.24v to 15v 0.25 ? typical output impedance pin to pin compatible with tlv431 precision voltage reference linear regulator controller secondary side controller for the low voltage power supply applications applications vo = vref o o o o o 1 + p p p p p r1 r2
iru431l / iru431al 2 rev. 1.5 07/13/01 parameter sym test condition min typ max units reference voltage vref i k =10ma, v ka =vref, ta=25 c 1.228 1.240 1.252 v iru431l i k =10ma, v ka =vref 1.221 1.240 1.259 reference voltage vref i k =10ma, v ka =vref, ta=25 c 1.234 1.240 1.246 v iru431al i k =10ma, v ka =vref 1.228 1.240 1.252 vref deviation over full vref(dev) v ka =vref, i k =10ma 6 mv temperature range note 1 ratio of vref change to ? vref/ ? v ka i k =10ma, dv ka =vref to 6v -1 -6 mv/v cathode voltage change reference pin current i k =10ma, r1=10k ? , r2=open 0.15 1 a iref deviation over full iref(dev) i k =10ma, r1=10k ? , r2=open 0.05 a temperature range note 1 minimum cathode current i k(min) v ka =vref 55 80 a off state cathode current ioff v ka =6v, vref=0v 0.6 0.75 a v ka =10v, vref=0v 1.8 5 v ka =15v, vref=0v 3.2 10 dynamic impedance z ka 0 v ka =vref, f<1khz, 0.25 0.4 ? i k =0.1 to 15ma, note 2 unless otherwise specified, these specifications apply over ta=0 to 70 c, co =1 f. typical values refer to ta=25 c. low duty cycle pulse testing is used which keeps junction and case temperatures equal to the ambient tempera- ture. absolute maximum ratings input voltage (vin) .................................................... 15v continuous cathode current range .......................... -15ma to +15ma reference current range .......................................... -0.05ma to 1ma storage temperature range ...................................... -65 c to 150 c operating junction temperature range ..................... 0 c to 150 c package information 5-pin sot-23 (l5) 3-pin sot-23 (l3) 8-pin plastic soic (s) ja = 450 c/w ja = 450c/w ja =160 c/w electrical specifications note 1: the deviation parameters, vref(dev) and iref(dev) are defined as the differences between the maximum and the minimum values obtained over the rated tem- perature range. the average full range temperature coef- ficient of the reference input voltage is defined as: where: vref unit is ppm/ c ? t a is the rated operating free air temperature of the device. vref can be positive or negative depending on whether minimum vref or maximum vref respectively occurs at the lower temperature. cathode ref anode 1 2 3 top view anode ref nc nc cathode 3 2 1 4 5 top view nc nc anode anode nc nc ref cathode 4 3 2 1 5 6 7 8 top view vref = vref(dev) vref(25 c) o o o o o p p p p p 10 6 ? t a
iru431l / iru431al 3 rev. 1.5 07/13/01 pin descriptions block diagram figure 2 - simplified block diagram of the iru431l resistors from the ref pin to the cathode pin and to ground form a divider that sets the output voltage. the output of the shunt regulator. a capacitor of 1 f minimum value must be connected from this pin to anode pin to insure unconditional stability. ground pin. this pin must be connected to the lowest potential in the system and all other pins must be at higher potential with respect to this pin. these pins are not connected internally. note 2: the dynamic impedance when v ka =vref is de- fined as: cathode 1.24v anode ref + when the device is operating with two external resistors (see figure 3), the total dynamic impedance of the cir- cuit is given by: sot-23 5-pin pin# sot-23 3-pin pin# 8-pin soic pin# pin symbol pin description 4 3 5 1, 2 1 2 3 na 8 1 3, 6 2, 4, 5, 7 ref cathode anode nc z ka 0 = ? v ka ? i k z ka = = z ka 0 o o o o o 1 + p p p p p r1 r2 ? v ? i
iru431l / iru431al 4 rev. 1.5 07/13/01 application information the maximum value for the biasing resistor is calcu- lated using the following equations: where: v min = minimum supply voltage i l(max) = maximum load current i b(max) = maximum bias current i k(min) = maximum value for the minimum cathode current spec i r = current through r1 assuming r1 = 2k ? as before, the maximum power dissipation of the resistor is calculated under the maximum supply voltage as follows: thermal design the iru431l is offered in the plastic 8-pin soic or the surface mount sot-23 (l) packages. the 8-pin soic package has the maximum power dissipation capability of 775mw at ta=25 c with the derating factor of -6.2mw/ c. the sot-23 package has the maximum power dis- sipation capability of 150mw at ta =25 c with the der- ating factor of -1.2mw / c. output voltage setting the iru431l can be programmed to any voltages in the range of 1.24 to 15v with the addition of r1 and r2 external resistors according to the following formula: the iru431l keeps a constant voltage of 1.240v be- tween the ref pin and ground pin. by placing a resistor r2 across these two pins a constant current flows through r2, adding to the iref current and into the r1 resistor producing a voltage equal to: which will be added to the 1.240v to set the output volt- age as shown in the above equation. since the input bias current of the ref pin is 0.5 a max, it adds a very small error to the output voltage and for most applica- tions can be ignored. for example, in a typical 5v to 3.3v application where r2=1.21k ? and r1=2k ? the error due to the iadj is only 1mv which is about 0.03% of the nominal set point. figure 3 - typical application of the iru431l for programming the output voltage. biasing resistor (r b ) selection the biasing resistor r b is selected such that it does not limit the input current under the minimum input supply and maximum load and biasing current. an example is given below on how to properly select the biasing resistor. assuming: 431app2-1.0 v r2 co r1 r iru431l i r k i l v ka = v o b in l p r b (max) = where: v max = maximum supply voltage p r b (max) = maximum rb power dissipation p r b (max) = = 73mw (6 - 3.3) 100 2 (v max - v ka ) r b 2 r b(max) = i b(max) = i k(min) + i r v min - v ka i b(max) + i l(max) i r = = 1.03ma i b(max) = 0.08 + 1.03 = 1.11ma r b(max) = = 108 ? selecting r b = 100 ? 3.3 - 1.24 2 4.5 - 3.3 1.11 +10 v min = 4.5v v max = 6v v ka = 3.3v i l = 10ma vo = v ka = vref o o o o o 1 + p p p p p + iref r1 r1 r2 (1.240 / r2) r1 + iref r1
iru431l / iru431al 5 rev. 1.5 07/13/01 in our previous example, the maximum power dissipa- tion of the device is calculated under no load and maxi- mum input supply condition. the maximum power is calculated using the following equation: as shown in the power dissipation table, both packages can handle this power dissipation. the table below summarizes the maximum power dissi- pation capability of each package versus ambient tem- perature. ambient temperature (ta) - c pkg 25 40 50 60 70 8-pin soic 775mw 682mw 620mw 558mw 496mw sot-23 150mw 132mw 120mw 108mw 96mw p max = v ka o o o o o p p p p p where: p max = maximum power dissipation of the 431l for our example: p max = 3.3 o o o o o p p p p p = 89mw v max - v ka r b 6 - 3.3 100 stability the iru431l has many different domains of stability as a function of the cathode current. as is typical of three- terminal shunt regulators, the iru431l has many do- mains of stability. the actual domain in which any prac- tical circuit operates is related to cathode current. in general the device will be unconditionally stable for any cathode current if a capacitor, 1 f or larger, is connected between the cathode and the anode. if the cathode cur- rent is always higher than 3ma under minimum line and maximum load conditions, the capacitor value can be reduced to 0.01 f and the system will be stable.
iru431l / iru431al 6 rev. 1.5 07/13/01 typical application i740 application figure 4 - low cost 3.3v to 2.7v output for intel i740 application. ref desig description qty part # manuf u1 shunt regulator 1 iru431l ir c1,2 capacitor 2 elect,220 f,6.3v,ecaojfq221 panasonic r1 resistor 1 6.2k ? , 5%, smt r2 resistor 1 118 ? , 1%, smt r3,r4 resistor 2 100 ? , 1% smt hs1 heat sink use minimum of 1" square copper pad area for load current <4a ir world headquarters : 233 kansas st., el segundo, california 90245, usa tel: (310) 252-7105 tac fax: (310) 252-7903 visit us at www.irf.com for sales contact information. data and specifications subject to change without notice. 02/01 431app3-1.2 12v r3 r2 r1 u1 v out v in c1 q1 c2 r4
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